As is known in the art, a frequency modulated continuous wave (FMCW) radar transmits a radar signal at a transmitted frequency that is continuously changing. In order to identify a range to a target, the FMCW radar measures a difference in frequency between a received radar signal, which is returned as an echo from the target, and the transmitted frequency. The difference in frequency is associated with a time delay between the transmitted signal and the received signal, i.e., a time that it takes the transmitted signal to reach the target and to return back to the radar.
In typical FMCW radar, for example, the frequency of the transmitted FMCW signal linearly increases from a first predetermined frequency to a second predetermined frequency in a so-called “chirp” signal. The chirp signal is often repeated at a repetition rate. FMCW radar has the advantages of high sensitivity, relatively low transmitter power, and good range resolution. In one conventional FMCW radar, the chirp signal varies substantially linearly from approximately 24.05 GHz to approximately 24.25 GHz.
A conventional FMCW radar uses a mixer, which mixes (i.e., multiplies) the transmitted and received signals. One of the outputs of the mixer is the above-described difference in frequency between the transmitted and received signals, which is also referred to herein as a “downconverted signal” or a “video signal”, which can have a “beat frequency.” The downconverted signal occurs at a frequency substantially lower than the frequency of the transmitted or received signals. The downconverted signal can be time sampled, for example, with an analog-to-digital (A/D) converter, and the time samples can be converted to the frequency domain, for example, with a fast Fourier transform (FFT) to provide a frequency spectrum. From the frequency spectrum, a variety of techniques can be used to identify the downconverted signal associated with range to the target. Some such techniques are described in U.S. Pat. No. 6,577,269, issued Jun. 10, 2003.
It will be appreciated that the frequency spectrum contains not only the downconverted signal corresponding to range to the target, but also contains noise. The noise is associated with a variety of noise sources including, but not limited to, electrical (i.e., thermal) noise sources and radar signal noise sources that may be present in the environment in which the FMCW radar is used. It will also be appreciated that there can be more than one target in a field of view of the radar system. Therefore, the time samples of the downconverted signal can include more than one beat frequency.
In order to locate a range to the target from the frequency spectrum, a frequency signal within the frequency spectrum is identified, the frequency of which is indicative of a range to the target. However, some types of interfering radar signals can greatly degrade the ability to find the frequency signal associated with the target within the frequency spectrum. For example, an interfering radar signal at sufficiently high power level and within the swept band (i.e., within the chirp frequency limits) of the FMCW radar can corrupt the time samples of the downconverted signal to such an extent that the resulting frequency spectrum is overwhelmed by the interfering signal and so the frequency signal associated with the target cannot be found in the frequency spectrum.
Referring now to FIG. 1, a graph 10 has a horizontal axis in units of frequency provided as FFT frequency bins and a vertical axis in units of dB in FFT counts (provided by FFT processing of time samples of the downconverted signal). A curve 12 has a peak 14 indicative of a beat frequency, f1, and a corresponding range to a target. The curve 12 also has a noise background 16. A curve 18 has no distinct peak that is clearly characteristic of a target. The curve 18 is indicative of the output of the FFT frequency domain processing when a received signal represented by the curve 12 also includes an interfering signal.
It will be appreciated that, even where the interfering signal is at a single frequency, the resulting processing of the FMCW radar system, including the above-described mixing, and the above-described FFT processing, results in a smearing of the single interfering signal frequency throughout the frequency spectrum. This is due to the fact that the mixing process provides a mixing output signal corresponding to the interfering signal for only a brief time. An apparent increase in the noise level across some or all of the frequency spectrum reduces the signal to noise ratio (SNR) of the peak 14 (i.e., of the target) and greatly reduces the probability of detection of the FMCW radar. In the curve 18, either the peak 14 cannot be found, or the peak 14 cannot be accurately found.
One particular application of the FMCW radar is in an automobile radar system, for example, used to detect an object in a blind spot next to a vehicle. Automobile radars often use the above-described frequency chirp extending, for example, from approximately 24.05 GHz to approximately 24.25 GHz. Conventional police radars used, for example, to detect speed of vehicles, operates within this band, for example, at approximately 24.197 GHz. In automobile applications, it is necessary to provide a radar system capable of accurately and reliably detecting objects, e.g., other vehicles, with minimal influence from interfering signals.
Accuracy and reliability of the radar system are very important. Characteristics of the vehicle radar system that contribute to accuracy and reliability include susceptibility of the sensor to noise, including interfering signals, and the overall precision with which received radio frequency (RF) signals are processed in the presence of the noise and interfering signals to detect objects. Susceptibility to noise, including interfering signals, can cause a vehicle radar system to falsely detect an object (i.e., to raise a false alarm rate), and/or, can cause the vehicle radar system to miss a detection of an object (i.e., to have a reduced probability of detection).